Geothermal heat originating from the earth's core is a “renewable” source of energy. The magma below the earth's crust heats nearby rocks and water. Hot geothermal effluent comprised of water and steam travels through faults and cracks to the earth's surface as geysers. Geysers in northern California are currently sources for geothermal energy plants. However, most hot geothermal water stays deep underground trapped in geothermal reservoirs and must be brought to the surface via a drilled production well if it is to be productively used.
A geothermal power plant is a thermal plant which uses geothermal resources as its principal source for generation of electrical power. A geothermal effluent is the effluent which is heated by geothermal heat and used to power the geothermal power plant. There are basically three kinds of geothermal power plants which utilize this hot water/steam in geothermal reservoirs as a geothermal effluent. A “dry” steam reservoir produces steam, but very little water. This steam is piped directly to a “dry” steam power plant to provide the force to spin a turbine generator.
A geothermal reservoir that produces mostly hot water ranging in temperature from 300-200 degrees F. is used in a “flash” power plant. This high temperature water is brought to the surface and released from the pressure of the reservoir; flashing into steam in a separator. The steam drives turbines.
In a third kind of plant, called a “binary” power plant, geothermal water between 250-360 degrees F. is passed through a heat exchanger where its heat is transferred to a second (binary) liquid that boils at a lower temperature than water.
When heated, the binary liquid flashes to vapor which expands and spins turbine blades.
In all of these types of plants, the geothermal effluent comprised water or steam may contain varying amounts of hydrogen sulfide (H2S) which, if not properly disposed, may cause environmental, health, and safety problems. In the “dry” and “flash” plants, the exhaust from the power plant turbine generally passes through a condenser which produces a condensate stream and a non-condensable gas stream which contain varying amounts of hydrogen sulfide (H2S). In the binary plant, the “cool” geothermal fluid is re-injected into the production formation. This fluid contains hydrogen sulfide and may be treated prior to re-injection.
Typical ways of removing hydrogen sulfide from geothermal effluent are by injecting oxidizing agents such as peroxides or hypochlorite; using an amine removal system; absorption into water; or injecting triazine based products. Oxidizing agents are corrosive and may result in formation of solids that may cause operational problems. Amine systems are efficient, but require a large capital investment. Absorption of hydrogen sulfide into water streams results in toxic water that has the potential to subsequently release hydrogen sulfide into the atmosphere. Triazines are inefficient when injected directly into an aqueous system and require large volumes of chemical to be effective or require large capital investments if used on non-condensed steam.